Effect of the nitrogen-to-hydrogen ratio on the mechanical behavior of vanadium, niobium and tantalum

In niobium and tantalum alloys containing nitrogen-to-hydrogen ratios greater than unity, no hydrides were observed down to 78 K, the lowest temperature investigated. In vanadium alloys with a nitrogen-to-hydrogen ratio greater than unity, hydrides were observed but the hydride precipitation temperature was drastically decreased from that in the alloys containing hydrogen alone or having a nitrogen-to-hydrogen ratio less than unity. In vanadium the strenght and ductility appeared to be influenced by the magnitude of the nitrogen-to-hydrogen ratio. In niobium and tantalum the resultant strengthening from combined nitrogen and hydrogen seemed to be a superposition of the strengthening resulting from the individual contributions of nitrogen and hydrogen regardless of the nitrogen-to-hydrogen ratio (about 0.27 or 1.3). Similarly, the loss of ductility and the ductile-to-brittle transition temperature (DBTT) of niobium and tantalum were primarily a consequence of the hydrogen content and they were not significantly affected by the nitrogen-to-hydrogen ratio or the absence of hydrides when the nitrogen-to-hydrogen ratio was greater than unity. In vanadium the strengthening that occurred when the nitrogen-to-hydrogen ratio was 1.38 was about 25%–75% greater, depending on the temperature, than that resulting when the nitrogen-to-hydrogen ratio was 0.45 or when the individual contributions of nitrogen and hydrogen to strengthening were added. In addition, the DBTT for vanadium with a hydrogen-to-hydrogen ratio of 1.38 was significantly below that expected on the basis of the hydrogen content or the results for with a nitrogen-to-hydrogen ratio of 0.45.